Add apply_xmap() methods to conformr

[cynthiahqy]

Implement a two interfaces, powered by underlying internal function:

• data first, xmap second pipeline interface: apply_xmap()
• xmap first, data second piepeline interface xmap_transform()
• internal xmap + data functions: transform_data_with_xmap() -- with full validation & lightweight (i.e. class check only)

Sample user code for apply_xmap() -- need to decide on arg names.

conformr::apply_xmap(data_in = state_data, xmap = agg_map,
                     in_codes = adm1, in_values = Pop,
                     out_codes = NULL, out_values = NULL)